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Over 40 cd/A Efficient Green Quantum Dot Electroluminescent Device Comprising Uniquely Large-Sized Quantum Dots Ki-Heon Lee,† Jeong-Hoon Lee,† Hee-Don Kang,† Byoungnam Park,† Yongwoo Kwon,† Heejoo Ko,‡ Changho Lee,‡ Jonghyuk Lee,‡ and Heesun Yang†,* †
Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea and ‡Display Research Center, Samsung Display Co., Ltd. Yongin, Kyunggi-do 446-811, Korea
ABSTRACT Green CdSe@ZnS quantum dots (QDs) of 9.5 nm size with a
composition gradient shell are first prepared by a single-step synthetic approach, and then 12.7 nm CdSe@ZnS/ZnS QDs, the largest among ZnS-shelled visibleemitting QDs available to date, are obtained through the overcoating of an additional 1.6 nm thick ZnS shell. Two QDs of CdSe@ZnS and CdSe@ZnS/ZnS are incorporated into the solution-processed hybrid QD-based light-emitting diode (QLED) structure, where the QD emissive layer (EML) is sandwiched by poly (9-vinlycarbazole) and ZnO nanoparticles as hole and electron-transport layers, respectively. We find that the presence of an additional ZnS shell makes a profound impact on device performances such as luminance and efficiencies. Compared to CdSe@ZnS QD-based devices the efficiencies of CdSe@ZnS/ZnS QD-based devices are overwhelmingly higher, specifically showing unprecedented values of peak current efficiency of 46.4 cd/A and external quantum efficiency of 12.6%. Such excellent results are likely attributable to a unique structure in CdSe@ZnS/ZnS QDs with a relatively thick ZnS outer shell as well as a wellpositioned intermediate alloyed shell, enabling the effective suppression of nonradiative energy transfer between closely packed EML QDs and Auger recombination at charged QDs. KEYWORDS: quantum dots . electroluminescence . current efficiency
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luorescent semiconductor quantum dots (QDs) have shown great promise as visible emitters for the fabrication of electroluminescent (EL) QD-based lightemitting diodes (QLEDs). Enabled by the exceptionally color-pure emission of wellsynthesized Cd-based IIVI QDs, QLEDs possess a superior color gamut property compared to state-of-the-art organic LEDs (OLEDs). To catch up with the performance level, in particular, device efficiency, of technology-mature OLEDs, significant efforts have been dedicated to improving QLED efficiency mainly by searching new types of charge-transport layers (CTLs) and/or altering the conventional device architecture.119 In a prototypical QLED structure, the QD emitting layer (EML) of multiple monolayers (MLs) comes into direct contact with two CTLs of hole transport layer (HTL) and electron LEE ET AL.
transport layer (ETL) for the efficient injection of charge carriers to the active QD region from the respective electrodes. Among the combinations of HTL/ETL, hybrid devices with organic HTL (e.g., poly(N,N0 -bis(4-butylphenyl)-N,N0 -bis(phenyl)benzidine) (poly-TPD), poly[(9,9-dioctylfluorenyl-2,7diyl)-co-(4,40 -(N-(4-sec-butylphenyl))diphenylamine) (TFB), poly(9-vinlycarbazole) (PVK)), and inorganic metal oxide ETL (e.g., ZnO, TiO2)19 have recently gained a significant attention since in terms of efficiency and luminance they outperformed the devices comprising either organic or inorganic CTLs.1017 For example, in 2011, Kim et al. reported red (R), green (G), and blue (B) hybrid QLEDs consisting of a transferprinted QD layer with a HTL of TFB and an ETL of solgel-derived TiO2, showing peak luminances of 16380, 6425, and 423 cd/m2, VOL. XXX
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* Address correspondence to
[email protected]. Received for review February 11, 2014 and accepted April 23, 2014. Published online 10.1021/nn500852g C XXXX American Chemical Society
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recently, our group demonstrated a high-efficiency blue QLED with the peak values of a CE of 2.2 cd/A and an EQE of 7.1% by employing newly developed ternary blue (452 nm) CdZnS/ZnS QDs comprising an alloyed core/shell interface and thick shell.8 Herein, green-emitting, chemical composition gradient-shell CdSe@ZnS QDs are first synthesized, and then thicker shell CdSe@ZnS/ZnS QDs consisting of an additional 1.6 nm thick ZnS shell, leading to an uniquely large size of 12.7 nm, are consecutively prepared. The fluorescent stability behaviors of individual QDs against a repeated cycle of purification and ligand exchange are compared. Even though the inverted structure QLED has been claimed to be superior with respect to luminance and efficiency to the conventional counterpart, i.e., ITO anode and Al cathode,18,21 our study rather sticks to the latter to fulfill the fabrication of all solution-processed hybrid QLED consisting of poly(9-vinlycarbazole) (PVK) HTL and ZnO NP ETL. Two QLEDs integrated with CdSe@ZnS versus CdSe@ZnS/ZnS QDs are fabricated and their EL characteristics are compared, resulting in drastic differences in luminance and efficiencies and thus implying the crucial impact of the presence of an additional ZnS shell on device performance. Highly color-saturated green CdSe@ZnS/ZnS QD-based device displays the unprecedentedly high peak efficiencies of a CE of 46.4 cd/A and an EQE of 12.6%, whose values are more than 2 times higher compared to the best G-QLED reported earlier.18
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respectively, and peak power efficiency (PE) of 4.25 lm/W from R-QLED.3 Later, Holloway et al. demonstrated fully solution-processed, higher performance R-, G-, B-devices with peak luminances of 31000, 68000, and 4200 cd/m2 and peak current efficiencies (CEs) of 3.9, 7.5, and 0.32 cd/A, respectively, by introducing a ZnO nanoparticle (NP) ETL with an HTL of poly-TPD.4 Since then, ZnO NP has become the most popular ETL material for QLED fabrication. Highly bright, efficient hybrid QLEDs have been further demonstrated by employing an inverted device architecture. In those devices, ZnO NP ETL and QD EML were successively spin-deposited on an indium tin oxide (ITO) cathode, and then 4,40 -bis(carbazole-9-yl)biphenyl (CBP) HTL, MoO3 hole injection layer (HIL), and Al anode were sequentially thermally evaporated. The completed R-, G-, B-QLEDs displayed peak luminances of 23040, 218000, and 2250 cd/m2 along with the peak CEs of 5.7, 19.2, and 0.4 cd/A, respectively.18 A year later, Mashford et al. reported R-QLEDs with a similar inverted structure but different HTL/HIL materials, and the record maximum values of a CE of 19 cd/A and an external quantum efficiency (EQE) of 18% were achieved simply through optimizing the QD EML thickness.19 However, all inverted structure QLEDs reported to date cannot be fully solution-processed, since the solution processing of organic HTL following QD deposition would cause a destructive interlayer mixing. The device efficiency of QLED is substantially limited by efficient nonradiative processes of inter-QD Förster resonant energy transfer (FRET) between a closely packed QD ensemble and multicarrier Auger recombination (AR) of charged QDs,20 the degrees of which are sensitively dependent on QD structure. Hence, persistent synthetic refinements toward QD structure rendering the above nonradiative events minimized, specifically by engineering size (or shell thickness) and/or core/shell interface,6,8,2022 have been made for the improvement of QLED efficiency. The luminance and EQE of QLED with giant-shell CdSe/CdS QDs with shells thicker than 10 MLs were found to be 1 order of magnitude higher than those of the thinshell QD device, even though the solution photoluminescence quantum yield (PL QY) of thin-shell QDs was much higher than that of giant-shell ones.20 This was attributed to the suppression of both FRET and AR processes in such large-sized QDs. Another powerful strategy for minimizing AR is to insert an intermediate alloyed layer at core/shell interface, affording a smooth confinement potential.23,24 Bae et al. utilized two redemitting CdSe/CdS and CdSe/CdSe0.5S0.5/CdS QDs with similar photophysical properties and the same size (∼14 nm) for QLED fabrication and found a beneficial effect of the interfacial alloyed layer (i.e., ∼1.5 nm thick CdSe0.5S0.5) on QLED efficiency as a result of the reduction of Auger decay rate.21 Also, very
RESULTS AND DISCUSSION Composition gradient-shell CdSe@ZnS QDs have been synthesized via a single-step method with an appreciably modified version from the protocol in the literature13,25 (see the Experimental Section for synthetic details). Co-injection of (SþSe)-trioctylphosphine (TOP) to a mixture of Cd(acet)2 plus ZnO in oleic acid (OA) at an elevated temperature (310 °C) successfully led to the spontaneous formation of CdSe-rich core @ZnS-rich shell structure with a radially smooth composition gradient, enabled by the higher reactivity and consequent faster consumption of Cd(OA)2 and SeTOP relative to Zn(OA)2 and S-TOP in the QD growth reaction.5,25 The emission wavelength of CdSe@ZnS QDs can be readily tuned to either the blue or red side through changing the relative molar ratios of Cd/Zn and/or Se/S. Instead, in this work, the fine tailoring of emission wavelength was achievable with the constant molar ratios in both cationic and anionic precursors but with the minute variation of the injected amount of anionic mixture of (SeþS)-TOP. Normalized PL spectra of CdSe@ZnS QDs synthesized by injecting different (SeþS)-TOP amounts of 1.92.1 mL are shown in Figure 1a, where a systematic blue-shift is observed with increasing the anion amount, specifically from 524 to 510 nm for 1.9 and 2.1 mL of (SeþS)-TOP, VOL. XXX
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respectively. This result may be correlated to anion amount-dependent QD nucleation frequency.26,27 That is, the introduction of a larger anion quantity would allow a more frequent nucleation and thus produce a larger number of QD nuclei, by which the QD growth is more limited and the relatively smaller QDs are obtained. PL QYs of a series of CdSe@ZnS were similar in the range of 4042%. The respective CdSe@ZnS QDs were consecutively overcoated with an additional ZnS shell, producing thicker-shell CdSe@ZnS/ZnS QDs (Figure 1c). The emission wavelengths of the resulting QDs, peaking at 507, 516, and 522 nm for the cases of 2.1, 2.0, and 1.9 mL of (SeþS)TOP, respectively (Figure 1b), were slightly blue-shifted by 23 nm compared to those of CdSe@ZnS ones. These blue-shifts directly result from the band gap widening as recognized from the comparison of absorption spectra (Figure 1Sa, Supporting Information), being attributable to the interfacial alloying throughout the shelling reaction. As shown in PL spectral comparison (Figure 1Sb, Supporting Information), the better surface passivation afforded by such an additional shelling led to about 2 fold-increase in PL QY of LEE ET AL.
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Figure 1. Normalized PL spectra of (a) CdSe@ZnS and (b) CdSe@ZnS/ZnS QDs systematically emission-tuned by slightly varying the injected amount (1.92.1 mL) of anionic mixture of (SeþS)-TOP. (c) Schematic illustration of CdSe@ZnS/ZnS QD sequentially comprising composition gradient-shell and additional ZnS outer shell. (d) Photographic image of strongly fluorescent (516 nm) CdSe@ZnS/ ZnS QD dispersion under UV irradiation.
7983%. Also the emission bandwidths (21 nm) of CdSe@ZnS/ZnS QDs became a little narrower as compared to those (23 nm) of their CdSe@ZnS counterparts. In multicompositional QD systems such as the present quaternary CdZnSeS QDs, both chemical composition and size of the QDs jointly determine the band gap energy (or emission wavelength), and their dispersion degrees directly influence the emission bandwidth. Assuming that the actual core sizes in CdSe@ZnS QDs would remain invariant after an additional shelling, the emission narrowing observed (Figure 1b) is presumably ascribable to the compositional homogenizing assisted by the aforementioned interfacial alloying. The representative 516 nm-emitting CdSe@ZnS/ZnS QD sample prepared with 2.0 mL of (SeþS)-TOP, exhibiting a strong fluorescence under UV irradiation, is shown in Figure 1d. In most of visible-emitting core/shell QDs, ZnS is the most commonly chosen shell composition, since its sufficiently high band gap provides large electron/hole band offset potentials in the form of type I electronic structure. Except for our recently developed blue CdZnS/ZnS QDs8 the sizes of such heterostructured QDs reported to date are below 10 nm irrespective of their visible emission wavelengths, indicative of the synthetic difficulty of larger-sized QDs with thicker ZnS shells. Although giant-shell CdSe/CdS QDs with ultrashell thickness up to 16 MLs were proposed as promising alternatives to the conventional thin-shell counterparts for minimizing the photoblinking via an AR suppression,28,29 the compositional combination of these giant-structured QDs has been confined to the CdSe core/CdS shell only, since they show a minimal lattice mismatch required for a thick shell growth. Unfortunately, these excessively thick-shell QDs exhibited only moderate QY values due to the generation of more internal defects with increasing CdS shell.29 And such a given compositional system of CdSe/CdS provides a quasi-type II electronic structure, where a hole is strongly confined into core, while an electron is extensively delocalized into shell, lowering the confinement energy and consequent excitonic transition energy. Hence, the emission wavelength of giant-shell CdSe/CdS QDs has been tailored mainly by varying CdS shell thickness with the size of CdSe core almost fixed, but the resulting spectral tunability was quite narrow within a primary window of red emission.20,28,29 The average size of our CdSe@ZnS QDs was estimated to be 9.5 nm based on high-resolution transmission electron microscopic (TEM) image (Figure 2a). And CdSe@ZnS/ ZnS QDs obtained through the consecutive ZnS shelling possessed a bigger size of 12.7 nm by an additional overgrowth of 1.6 nm-thick ZnS (corresponding to ∼5.2 MLs) (Figure 2b). This size difference could also be readily noticed from low-magnification TEM images (Figure S2, Supporting Information). If it is consdiered that the gradient-shell composition of CdSe@ZnS
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ARTICLE Figure 2. High-resolution TEM images of (a) CdSe@ZnS and (b) CdSe@ZnS/ZnS QDs with the average sizes of 9.5 and 12.7 nm, respectively (scale bar, 5 nm).
QDs would consist of gradually increasing Zn and S contents with a QD radius and finally a pure ZnS outward, the actual thickness of ZnS shell in CdSe@ ZnS/ZnS QDs should be thicker than 1.6 nm. It is worth noting that the present CdSe@ZnS/ZnS QDs are the largest ones as compared with ZnS-shelled R, G, and B QDs reported previously. An initial strong PL of as-prepared QDs is usually not retained after a thorough purification cycle, typically attributable to the gradual physical detachment of surface capping ligands with the repetition number of purification. To address this QY reduction concomitant to the purification step, the replacement of labile TOP ligand with more strongly binding thiol13 or oleate species8,18 and the structural improvement of core/shell QDs with multishell6,30 or giant-shell29 have been effectively attempted. Figure S3a (Supporting Information) presents the comparison of PL QY variations of 2.0 mL (SeþS)-TOP-based QDs without versus with an additional ZnS shell against a purification cycle with the repeated round up to 8 times. CdSe@ZnS QDs suffered from an archetypal QY deterioration as the purification was repeated. The QY of 8-purified QD sample reached only 24%, corresponding to a 43% drop relative to the original QY. In contrast, CdSe@ZnS/ZnS QDs displayed an excellent fluorescent stability, retaining the initial high QY (83%) even after an 8-purification cycle. A sufficiently thick ZnS shell may serve as an effective physical barrier in preventing the electron/ hole wave functions of core domain from leaking into QD surface trap sites, rendering the excitonic recombination immune to the undesirable change of QD surface environment (e.g., ligand loss). A hydrophobicto-hydrophilic ligand exchange of QDs is a common process to obtain water-soluble QDs required for biological applications. In general, PL of ligand-exchanged QDs is markedly quenched, usually attributable to the creation of nonradiative trap sites by the ligand exchange-involved devastation of QD surface. Hydrophobic LEE ET AL.
surface ligands of CdSe@ZnS/ZnS QDs were readily exchanged with a hydrophilic mercaptopropionic acid (MPA) (see the Supporting Information for a detailed ligand-exchange procedure). In contrast to the previous results on PL reduction of ligand-exchanged QDs,30,31 PL intensity of MPA-capped CdSe@ZnS/ZnS QDs well maintained that of its hydrophobic counterparts (Figure S3b, Supporting Information), again supporting that the presence of thick ZnS shell enables QDs to possess a high fluorescent stability against the environmental change of QD surface. Although core/shell structured QDs usually possess excellent QYs in solution state, their QYs in the form of closely packed thin film become severely reduced as a result of the efficient nonradiative FRET, typically retaining
